The graph AB shown in figure is a plot of temperature of a body in degree Celsius and degree Fahrenheit.  Then: 
 

Relation between Celsius and Fahrenheit scale of  temperature is 
 $\frac{C}{5}=\frac{F-32}{9}$
 $\Rightarrow C=\frac{5}{9}F-\frac{160}{9}$
Equating above equation with standard equation of line  $y=mx+c$  we get slope of the line 
AB is   $m=\frac{5}{9}$